JP6532748B2 - Multicore fiber - Google Patents

Description

  The present invention relates to a multicore fiber and is suitable for improving the degree of freedom in design.

  At present, the optical fiber used in the optical fiber communication system in widespread use in general has a structure in which the outer periphery of one core is surrounded by a clad, and information is transmitted by propagating an optical signal in this core Be done. In recent years, with the spread of optical fiber communication systems, the amount of information to be transmitted has dramatically increased. With the increase in the amount of information to be transmitted, large-capacity long-distance optical communication is being performed in optical fiber communication systems by using many optical fibers such as several tens to several hundreds. .

  In such an optical fiber communication system, it is known to transmit a plurality of signals by light propagating through each core using a multi-core fiber in which the outer peripheries of the plurality of cores are surrounded by one clad.

  Patent Document 1 listed below describes an example of a multi-core fiber. In this multi-core fiber, one core is disposed at the center of the cladding, and six cores are disposed around the centrally disposed core. Such an arrangement is a structure capable of close-packing the cores, so that many cores can be arranged for a specific clad outer diameter. Further, in the multi-core fiber described in Patent Document 1, in order to suppress cross talk of light propagating in each core, propagation constants of light propagating in cores adjacent to each other are different from each other.

  However, there is a demand to further suppress crosstalk as compared with the case of changing the effective refractive index of adjacent cores as in the multi-core fiber described in Patent Document 1. Therefore, there is known a multi-core fiber in which a low refractive index layer having a refractive index lower than that of a clad is disposed so as to surround the outer peripheral surface of each core, and crosstalk is further prevented. Patent Document 2 below describes such a multi-core fiber. Since the low refractive index layer is in the form of a trench when the multicore fiber is viewed from the viewpoint of the refractive index, the multicore fiber is referred to as a trench type, and the configuration from the core to the low refractive index layer is referred to as a core element. Even in such a trench-type multi-core fiber, it is preferable that propagation constants of light propagating through adjacent cores differ from each other in order to suppress crosstalk of light propagating through the respective cores.

JP 2011-170336 A JP 2012-118495 A

  However, as described above, in order to change the propagation constants of light propagating in the mutually adjacent cores, it is necessary to mutually change the refractive index and the diameter of the mutually adjacent cores. However, in order to perform communication with light of a desired mode in a desired wavelength band, the range of possible values of the refractive index and diameter of the core is narrow, and the refractive index and diameter of adjacent cores are mutually changed. There is a limit to the freedom of design.

  In addition, in the trench type multi-core fiber, when the core element is disposed so as to surround a specific core or core element, the light of high-order mode in the light propagating through the specific core or core element is difficult to escape and the cutoff The wavelength tends to be longer. Therefore, in order to suppress the propagation of light of modes higher than the mode of light propagating when the core element is present alone, the distance between the cores can not be made much smaller, and the freedom of design is also limited. .

  Then, this invention aims at providing the multi-core fiber which can improve the freedom degree of design.

  In order to achieve the above object, the present invention is a multi-core fiber for performing communication with light (x is an integer of 1 or more) up to the x-order LP mode in a communication band, and enclosing a plurality of cores and the plurality of cores And a cladding having a refractive index lower than the refractive index of the plurality of cores, and a coating layer covering the cladding and having a refractive index higher than the refractive index of the cladding. The plurality of cores propagate light up to the (x + 1) th order LP mode, respectively. The inter-core distances are such distances that crosstalk of light to the x-order LP mode is -40 dB / km or less and crosstalk of light of the (x + 1) -order LP mode is -30 dB / km or more. The distance between the outermost core in the cladding and the covering layer is the excess loss due to absorption of light into the covering layer up to the x-order LP mode propagating in the outermost core. The distance becomes 001 dB / km or less, and the excess loss due to absorption of the light of the (x + 1) order LP mode propagating in the outermost core to the covering layer is 3 dB / km or more.

  According to such a multi-core fiber, each core is a core that propagates 1 LP mode higher-order light than a core that propagates only light up to the x-order LP mode. Containment can be strengthened. Therefore, compared with a multi-core fiber configured with a core that propagates only light in the x-order LP mode, it is possible to suppress light crosstalk in the x-order mode. Therefore, the design freedom of the core spacing and the design freedom such as the refractive index and the diameter of each core are improved as compared with the multi-core fiber composed of cores that propagate light up to the x-order LP mode.

  By the way, the effective core area of the light of the (x + 1) -order LP mode is larger than the effective core area of the light up to the x-order LP mode. Taking advantage of this, the excess loss due to absorption of light in the covering layer up to the x-order LP mode propagating in the outermost core becomes 0.001 dB / km or less, The distance between the outermost core in the cladding and the covering layer can be set so that the excess loss due to absorption of the propagating (x + 1) order LP mode light into the covering layer is 3 dB / km or more . In the multi-core fiber of the present invention configured as such, light in the (x + 1) -order LP mode unnecessary for communication propagating through the outermost core in the cladding is absorbed by the covering layer and lost. In addition, taking advantage of the fact that the effective core area of the light to the (x + 1) order LP mode is larger than the effective core area of the light of the x order LP mode, the distance between cores is the light to the x order LP mode Crosstalk of −40 dB / km or less and crosstalk of light of the (x + 1) th-order LP mode of −30 dB / km or more. Therefore, the crosstalk of light to the x-order LP mode used for communication is suppressed, and the light of the (x + 1) -order LP mode, which is light unnecessary for communication, cross-talks. Accordingly, the (x + 1) -order light can move by crosstalk to the outermost core in the cladding and is absorbed by the covering layer as described above. Thus, while suppressing the propagation of light of the (x + 1) th mode unnecessary for signal transmission, crosstalk between the modes used for signal transmission can be improved by strengthening the confinement of the light up to the xth mode, and the core It is possible to improve the design freedom of the spacing and the design freedom of the refractive index and diameter of the core.

  In addition, in a part of the plurality of cores in the longitudinal direction, the plurality of cores may further include an extension portion which is elongated so that the diameter of the plurality of cores is narrowed. Preferably, the light is propagated, and the propagation of light of the (x + 1) th-order LP mode is suppressed. In this case, it is more preferable that the loss of light of the (x + 1) th-order LP mode be 20 dB or more in the extension portion.

  By providing such an extending portion, light in the (x + 1) -order LP mode unnecessary for signal transmission can be further lost, and light in the mode unnecessary for communication can be more appropriately eliminated.

  Also, x may be set to 1. According to the multi-core fiber having such a configuration, it is possible to achieve the multi-core fiber for single mode communication in which the crosstalk is improved as compared with the multi-core fiber using only the core that propagates only the light of the conventional fundamental mode.

  As described above, according to the present invention, a multicore fiber capable of improving the degree of freedom in design is provided.

FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction of a multi-core fiber according to a first embodiment of the present invention. It is a figure which shows the refractive index distribution of the core element in the multi-core fiber of FIG. The figure which shows the calculation result of the relationship between cladding thickness and the excess loss by absorption to the coating layer of light in case the relative refractive index difference with respect to the cladding of a core is 0.45%, and the radius of a core is 5.17 micrometers. It is. The figure which shows the calculation result of the relationship between cladding thickness and the excess loss by absorption to the coating layer of light in case the relative refractive index difference with respect to the cladding of a core is 0.46%, and the radius of a core is 5.20 micrometers. It is. The figure which shows the calculation result of the relationship between the cladding thickness and the excess loss by absorption to the coating layer of light in case the relative refractive index difference with respect to the clad of a core is 0.47%, and the radius of a core is 5.22 micrometers. It is. It is a figure which shows the calculation result of the relationship between the bending radius of multi-core fiber, and crosstalk. It is sectional drawing perpendicular | vertical to the longitudinal direction of the multi-core fiber concerning 2nd Embodiment of this invention. It is a figure which shows the refractive index distribution of the core element in the multi-core fiber of FIG. The figure which shows the calculation result of the relationship between cladding thickness and the excess loss by absorption to the coating layer of light in case the relative refractive index difference with respect to the clad of a core is 0.45%, and the radius of a core is 6.11 micrometers. It is. The figure which shows the calculation result of the relationship between cladding thickness and the excess loss by absorption to the coating layer of light in case the relative refractive index difference with respect to the cladding of a core is 0.46%, and the radius of a core is 6.12 micrometers. It is. The figure which shows the calculation result of the relationship between cladding thickness and the excess loss by absorption to the coating layer of light in case the relative refractive index difference with respect to the clad of a core is 0.47%, and the radius of a core is 6.12 micrometers. It is. It is a figure which shows the calculation result of the relationship between the bending diameter of multi-core fiber, and crosstalk. It is sectional drawing perpendicular | vertical to the longitudinal direction of the multi-core fiber concerning 3rd Embodiment of this invention. It is the figure which looked at the multi-core fiber of FIG. 13 from the side. It is a figure which shows the calculation result of the relationship between draw ratio and the propagation loss of the light of LP11 mode. It is sectional drawing perpendicular | vertical to the longitudinal direction of the multi-core fiber concerning the modification of this invention.

  Hereinafter, preferred embodiments of a multi-core fiber according to the present invention will be described in detail with reference to the drawings. In addition, the scale described in each figure may be different from the scale described in the following description for easy understanding.

First Embodiment
FIG. 1 is a view showing the appearance of the multi-core fiber according to the present embodiment. As shown in FIG. 1, the multi-core fiber 1 includes a plurality of core elements 10, a clad 20 surrounding each core element 10 without a gap, and a covering layer 30 covering the clad 20.

  One core element 10 is disposed at the center of the cladding 20 as a first layer core element. A plurality of core elements 10 are disposed on the outer peripheral side of the first layer core element 10 as a second layer core element, and a plurality of core elements 10 on the outer peripheral side of the second layer core element 10 is a third layer core element , And the plurality of core elements 10 are disposed as the fourth layer core element on the outer peripheral side of the third layer core element 10. In this embodiment, the core elements are arranged in 1 to 6-12-12 in four layers as described above. Furthermore, a triangular lattice can be drawn by a line connecting centers of core elements 10 adjacent to each other, and each core element 10 is disposed on each lattice point of the triangular lattice. Thus, the plurality of core elements 10 are closely packed.

  Moreover, each core element 10 has the same structure as each other. Each core element 10 is a low refractive index layer in which the core 11, the inner clad 12 surrounding the outer peripheral surface of the core 11 without any gap, and the outer peripheral surface of the inner clad 12 without any space And 13.

  FIG. 2 is a diagram showing the refractive index distribution of each core element 10 of the multi-core fiber 1 shown in FIG. As shown in FIG. 2, the refractive index of the core 11 of the core element 10 is higher than the refractive index of the inner cladding 12, and the refractive index of the low refractive index layer 13 is the refractive index of the inner cladding 12 and the refractive index of the cladding 20. It is lower than. Thus, when each core element 10 is viewed from the viewpoint of the refractive index, the low refractive index layer 13 has a groove shape, and each core element 10 has a trench structure. With such a trench structure, confinement of light propagating through the cores 11 of the multi-core fiber 1 can be strengthened. In the present embodiment, the refractive index of the inner cladding 12 is the same as the refractive index of the cladding 20.

  Since each core element 10 of the multi-core fiber 1 has such a refractive index, for example, the cladding 20 and each inner cladding 12 are made of quartz to which no dopant is added, and each first core 11 is The low refractive index layer 13 is made of quartz or the like to which a dopant to lower the refractive index such as fluorine is added.

In addition, each core element 10 propagates light of the LP01 mode and light of the LP11 mode. The light of the LP01 mode propagating through each core element 10 preferably has an effective core cross-sectional area A _eff of about 80 μm ^{2 at} a wavelength of 1550 nm from the viewpoint of connectivity with a standard single mode fiber. Here, the relative refractive index difference with respect to the cladding 20 of low refractive index layer 13 delta _t is -0.7%, the ratio _r 2 / _{r 1} and the radius _{r 2} of a radius _{r 1} and the inner cladding 12 of the core 11 In the case of 1.7, the relative refractive index difference Δ with respect to the cladding 20 of the core 11 and the radius r of the core 11 in the case where the effective core cross-sectional area _Aeff of the light of the LP01 mode of light with a wavelength of 1550 nm is 80 μm ² the combination of ₁ shown in Table 1.

In this case, the effective core cross-sectional area _{Aeff at} the wavelength 1550 nm of the light of the LP11 mode propagating through the core 11 is approximately 92 μm ² .

  Also, the refractive index of the covering layer 30 is higher than that of the cladding 20. The covering layer 30 has a property of absorbing light, and the light reaching the covering layer 30 from the cladding 20 is absorbed by the covering layer 30 and disappears. As a material which comprises such a coating layer 30, an ultraviolet curable resin can be mentioned, for example.

  Next, the relationship between the excess loss due to absorption into the covering layer 30 of the light propagating through the core element 10 disposed in the fourth layer which is the outermost periphery and the cladding thickness will be described. As shown in FIG. 1, the cladding thickness Tc means the distance from the center of the core 11 arranged at the outermost periphery to the outer peripheral surface of the cladding 20. In the present embodiment, the cladding thickness Tc is constant regardless of which core element 10 is arranged at the outermost periphery.

FIG. 3 shows that when the relative refractive index difference Δ of the core 11 to the clad 20 is 0.45% and the radius r _{1 of the} core 11 is 5.17 μm in FIG. FIG. 16 is a diagram showing the calculation result of the relationship with excess loss due to absorption into the layer 30. 3, the solid line, the ratio W / _{r 1} of the thickness W of the radius _{r 1} and the low-refractive index layer 13 of the core 11 shows a state of 0.8, and a broken line, the radius _{r 1} of the core 11 and the low the ratio W / _{r 1} of the thickness W of the refractive index layer 13 shows a state of 0.9, the dotted line, the ratio W / _{r 1} of the thickness W of the radius _{r 1} and the low-refractive index layer 13 of the core 11 Indicates a state of 1.0. Further, FIG. 4 shows that when the relative refractive index difference Δ of the core 11 to the clad 20 is 0.46% and the radius r _{1 of the} core 11 is 5.20 μm, the clad thickness Tc and the light covering layer 30 are It is a figure which shows the calculation result of the relationship with the excess loss by absorption of. 4, the solid line, the ratio W / _{r 1} of the thickness W of the radius _{r 1} and the low-refractive index layer 13 of the core 11 shows a state of 0.7, and a broken line, the radius _{r 1} of the core 11 and the low the ratio W / _{r 1} of the thickness W of the refractive index layer 13 shows a state of 0.8, the dotted line, the ratio W / _{r 1} of the thickness W of the radius _{r 1} and the low-refractive index layer 13 of the core 11 Indicates a state of 0.9. Further, FIG. 5 shows that when the relative refractive index difference Δ of the core 11 to the clad 20 is 0.47% and the radius r _{1 of the} core 11 is 5.22 μm, the clad thickness Tc and the light covering layer 30 are It is a figure which shows the calculation result of the relationship with the excess loss by absorption of. 5, the solid line, the ratio W / _{r 1} of the thickness W of the radius _{r 1} and the low-refractive index layer 13 of the core 11 shows a state of 0.6, and a broken line, the radius _{r 1} of the core 11 and the low the ratio W / _{r 1} of the thickness W of the refractive index layer 13 shows a state of 0.7, the dotted line, the ratio W / _{r 1} of the thickness W of the radius _{r 1} and the low-refractive index layer 13 of the core 11 Indicates a state of 0.8.

In the calculation of FIG. 3 to FIG. 5, the light of LP01 mode is the light of 1625 nm at the wavelength with the largest effective core cross-sectional area _Aeff in C band and L band, and the light of LP11 mode is C band and L The wavelength with the smallest effective core area _Aeff in the band is 1530 nm. Also, in general, optical fibers are laid without being laid straight. Therefore, in the calculation of FIGS. 3 to 5, the bending radius of the multi-core fiber is set to 140 mm.

  As shown in FIG. 3 to FIG. 5, it can be seen that in the LP01 mode light, the excess loss due to absorption to the covering layer 30 is 0.001 dB / km or less in the region where the cladding thickness Tc is approximately 31 μm or more. Considering that the propagation loss of a standard single mode fiber is 0.19 dB / km, the difference of 0.001 dB / km is a very small value. In addition, it can be seen that in the LP11 mode light, the excess loss due to the absorption to the covering layer 30 is 3 dB / km or more in the region where the cladding thickness Tc is approximately 31 μm or less. If the excess loss due to the absorption to the covering layer 30 is 3 dB / km, the power can be reduced to 1/1000 or less by propagating the light through the multicore fiber 1 for 10 km. Therefore, it is possible to suppress the propagation of light of the LP11 mode which is unnecessary for signal transmission.

In FIGS. 3 to 5, in the region where the cladding thickness Tc is approximately 31 μm or less, the excess loss due to the absorption of the LP01 mode light into the coating layer 30 is 0.001 dB / km or less, and the LP11 mode light coating layer It can be seen that there is a solution in which the excess loss due to absorption to 30 is 3 dB / km or more. As described above, in FIGS. 3 to 5, in the C band and L band, the wavelength of light of the LP01 mode is excessive loss due to absorption into the covering layer 30 at the wavelength at which the effective core area A _eff is the largest. The wavelength of light of the LP11 mode indicates an excess loss due to absorption into the covering layer 30 at the wavelength at which the effective core area A _eff is the smallest. Therefore, when propagating LP01 mode light and LP11 mode light in the same wavelength band, the LP01 mode light propagates by excess loss due to absorption in a small covering layer 30 that does not interfere with optical communication, and LP11 mode light the combination of the radius r ₁ of the clad thickness Tc and the core 11 can be sufficiently attenuated will be present.

  Therefore, in the clad 20 of the multi-core fiber 1 of the present embodiment, the LP01 mode in which the distance between each of the cores 11 arranged at the outermost side in the clad 20 and the covering layer 30 propagates the core 11 arranged at the outermost side Excessive loss due to absorption of light (first-order LP mode) to the covering layer 30 is 0.001 dB / km or less, and covering of light of the LP11 mode (second-order LP mode) propagating through the core 11 disposed outermost The distance by which the excess loss due to absorption into the layer 30 is 3 dB / km or more is set.

  For this reason, in the multi-core fiber 1 of the present embodiment, the light of the LP01 mode propagating through the core element 10 disposed at the outermost periphery is regarded as excessive loss due to absorption in the covering layer 30 which does not affect optical communication. The light of the LP11 mode propagating through the core element 10 disposed is significantly reduced in power due to excess loss due to absorption into the covering layer 30.

  Next, the relationship between the inter-core distance and the crosstalk will be described. The inter-core distance is the distance between the centers of the cores 11 adjacent to each other.

FIG. 6 is a view showing the calculation result of the relationship between the bending radius and the crosstalk of the multi-core fiber 1 of the present embodiment. In the multi-core fiber 1 having 31 core elements shown in FIG. 1, when the cladding diameter is 230 μm and the cladding thickness Tc is 31 μm, the inter-core distance Λ is 32 μm. Therefore, in the calculation of Figure 6, as 32μm inter-core distance lambda, the wavelength of light of LP01 mode and the wavelength using the wavelength of light in the LP11 mode in the calculation of FIGS. 3-5, the radius r ₁ of the core 11 The ratio r ₂ / r ₁ of the inner clad 12 to the radius r ₂ of the inner cladding 12 was the same as the conditions in the case of Table 1. In FIG. 6, the solid line indicates the radius r _{1 of the} core 11 and the thickness of the low refractive index layer 13 using the relative refractive index difference Δ and the radius of the core 11 with respect to the cladding 20 of the core 11 used in the calculation of FIG. It is the result of calculation which made ratio W / r ₁ with W 0.9. A broken line, the thickness W of the radius r ₁ and the low-refractive index layer 13 with a radius of relative refractive index difference Δ and the core 11 and the cladding 20 of the core 11 used in the calculation of Figure 4, further core 11 The ratio W / r ₁ is calculated to be 0.8. The dotted line, the thickness W of the radius r ₁ and the low-refractive index layer 13 with a radius calculated by the relative refractive index difference Δ and the core 11 and the cladding 20 of the core 11 used in FIG. 5, further core 11 The ratio W / r ₁ is calculated as 0.7.

As shown in FIG. 6, the crosstalk of the light of the LP01 mode can be smaller than -40 dB / km in any case. In addition, the crosstalk of LP11 mode light was greater than -30 dB / km in each case. That is, under the above conditions, if the distance between the cores is 32 μm, crosstalk of light in the LP01 mode can be made -40 dB / km or less, and crosstalk of light in the LP11 mode can be made -30 dB / km or more . Furthermore, in FIG. 6, in the C band and L band, the wavelength of light of the LP01 mode is the wavelength with the largest effective core area A _eff, and the wavelength of light of the LP 11 mode is the most effective core area A _eff. The smaller wavelength is being calculated. Therefore, when LP01 mode light and LP11 mode light are propagated in the same wavelength band, the crosstalk of LP01 mode light is set to a small value that does not affect optical communication, and the crosstalk of LP11 mode light is set to a large value. There will be an inter-core distance で き る that can.

  Therefore, in each of the inter-core distances マ ル チ コ ア of the multi-core fiber 1 of the present embodiment, the crosstalk of light in the LP01 mode (primary LP mode) is -40 dB / km or less, and the light in the LP11 mode (second LP mode) The distance is such that the crosstalk is -30 dB / km or more.

  For this reason, in the multi-core fiber 1 of the present embodiment, crosstalk of LP01 mode light propagating through each core element 10 is suppressed, but LP11 mode light propagating through each core element 10 moves due to crosstalk. . Therefore, light of the LP11 mode propagating from the first layer to the third layer can be moved to the core element 10 of the fourth layer which is the outermost periphery by crosstalk.

  As described above, according to the multi-core fiber 1 of the present embodiment, each core element 10 propagates one-mode higher-order light than a core element that propagates light of only the LP01 mode (first-order LP mode) Since the core element is a core element, the confinement of the LP01 mode light in the core 11 can be strengthened. Therefore, compared with the core that propagates only the light of the LP01 mode, it is possible to suppress the crosstalk of the light of the LP01 mode. For this reason, the design freedom of the distance between cores and the design freedom of the refractive index and the diameter of each core are improved more than the multi-core fiber which propagates only the light of the LP01 mode.

  Further, the distance between the outermost core 11 and the covering layer, that is, the clad thickness Tc, is 0. 0 when the excess loss due to absorption of light of the LP 01 mode propagating in the outermost core 11 is 30%. The size is made to be 001 dB / km or less, and the excess loss due to absorption of the light of the LP11 mode propagating through the core 11 disposed at the outermost side into the covering layer 30 is 3 dB / km or more. Therefore, light of the LP11 mode unnecessary for communication propagating through the outermost core 11 in the cladding is absorbed by the covering layer 30 and lost. Further, each inter-core distance Λ is a distance at which crosstalk of light in the LP01 mode becomes -40 dB / km or less and crosstalk of light in the LP11 mode becomes -30 dB / km or more. Therefore, crosstalk of light in the LP01 mode used for communication is suppressed, and light in the LP11 mode, which is light unnecessary for communication, cross talks. Therefore, light in the LP11 mode can move to the outermost core 11 in the cladding 20 by crosstalk. Thus, light of the LP11 mode propagating in the core 11 disposed on the inner circumferential side of the cladding 20 travels to the core 11 disposed on the outermost side of the cladding 20 and is absorbed by the covering layer 30. Therefore, according to the multi-core fiber 1 of the present embodiment, the crosstalk of the LP01 mode used for signal transmission can be improved while suppressing the propagation of light of the LP11 mode not used for signal transmission.

Second Embodiment
Next, a second embodiment of the present invention will be described in detail with reference to FIGS. 7 to 12. In addition, about the component the same as that of 1st Embodiment, or equivalent, the same referential mark is attached | subjected and the overlapping description is abbreviate | omitted unless it demonstrates in particular.

  FIG. 7 is a cross-sectional view perpendicular to the longitudinal direction of the multi-core fiber according to the present embodiment, and FIG. 8 is a view showing a refractive index distribution of core elements of the multi-core fiber 2 of FIG. As shown in FIG. 7 and FIG. 8, in the multi-core fiber 2 of the present embodiment, each core element 10 does not have the inner cladding 12 and the core 11 is directly surrounded by the low refractive index layer 13. It differs from the multi-core fiber 1 of the first embodiment.

Also in the present embodiment, each core element 10 propagates light of the LP01 mode and light of the LP11 mode. Also in the present embodiment, for the same reason as the first embodiment, it is preferable that the effective core cross-sectional area _Aeff of light of the LP01 mode propagating through each core element be larger than 80 μm ² . Here, when the relative refractive index difference _Δt of the low refractive index layer 13 with respect to the cladding 20 is −0.7%, the effective core cross-sectional area A _eff of light of the LP01 mode of light having a wavelength of 1550 nm is 80 μm ² Table 2 shows combinations of the relative refractive index difference Δ with respect to the cladding 20 of the core 11 and the radius r _{1 of the} core 11 in this case.

In this case, the effective core area _Aeff of light of the LP11 mode propagating through the core 11 is approximately 119 μm ² .

  Next, the relationship between the excess loss due to absorption into the covering layer 30 of light propagating through the core element 10 disposed in the outermost layer, the fourth layer, and the cladding thickness will be described in the same manner as in the first embodiment. .

FIG. 9 shows the excess due to absorption of the cladding thickness Tc and light into the covering layer 30 when the relative refractive index difference Δ of the core 11 to the cladding 20 is 0.45% and the radius of the core 11 is 6.11 μm. It is a figure which shows the calculation result of relationship with loss similarly to FIG. In FIG. 10, when the relative refractive index difference Δ of the core 11 with respect to the cladding 20 is 0.46% and the radius r _{1 of the} core 11 is 6.12 μm, Fig. 5 is a view showing the calculation result of the relationship with the excess loss due to the absorption of the same as in Fig. 4. Further, FIG. 11 shows that when the relative refractive index difference Δ of the core 11 to the cladding 20 is 0.47% and the radius r _{1 of the} core 11 is 6.12 μm, the cladding thickness Tc and the light covering layer 30 are FIG. 6 is a view showing the calculation result of the relationship with the excess loss due to the absorption of the same as FIG.

  Also in the calculations of FIGS. 9 to 11, the wavelength of the light of the LP01 mode is 1625 nm, and the wavelength of the light of the LP11 mode is 1530 nm as in the first embodiment. Further, for the same reason as in the first embodiment, in the calculation of FIGS. 9 to 12, the bending radius of the multi-core fiber is set to 140 mm.

As shown in FIGS. 9 to 11, also in the present embodiment, in the LP01 mode light, the excess loss due to absorption in the covering layer 30 is 0.001 dB / km or less in the region where the cladding thickness Tc is approximately 31 μm or more. The light of the LP11 mode has an excess loss of 3 dB / km or more due to absorption into the covering layer 30 in a region where the cladding thickness Tc is approximately 31 μm or less. Therefore, in the region where the cladding thickness Tc is approximately 31 μm, the excess loss due to the absorption of light in the LP01 mode into the covering layer 30 is 0.001 dB / km or less, and the excess due to the absorption of light in the LP11 mode into the covering layer 30 It can be seen that there is a solution in which the loss is 3 dB / km or more. In the calculations of FIGS. 9 to 11, in the C band and L band, the wavelength of light of the LP01 mode is the wavelength at which the effective core cross-sectional area A _eff is the largest, and the wavelength of light of the LP 11 mode is the effective core cross-sectional area A _eff is the smallest wavelength. Therefore, the light of LP01 mode propagates by excess loss due to absorption to the small covering layer 30 that does not interfere with optical communication, and cladding thickness Tc capable of sufficiently attenuating light of LP11 mode unnecessary for communication and radius r _{1 of} core 11 There will be a combination of

  Therefore, the clad thickness Tc of the multi-core fiber 2 of the present embodiment is the core 11 disposed at the outermost side in the clad 20, as in the multi-core fiber 1 of the first embodiment propagating the core 11 disposed at the outermost side. And the excess layer due to the absorption of light of the LP 01 mode propagating in the outermost core 11 into the outer sheath 30 is 0.001 dB / km or less, The distance by which the excess loss due to absorption of light of the LP11 mode propagating through 11 into the covering layer 30 is 3 dB / km or more.

  Next, the relationship between the inter-core distance and the crosstalk in the present embodiment will be described.

FIG. 12 is a view showing the calculation result of the relationship between the bending radius and the crosstalk of the multi-core fiber 2 of the present embodiment. In the multi-core fiber 1 having 31 core elements shown in FIG. 7, the wavelength of light in the LP01 mode and the wavelength of light in the LP11 mode are set from FIG. 9 with an inter-core distance Λ of 32 μm as in the first embodiment. It was set as the wavelength used for calculation of FIG. 12, the solid line, by using the radius calculated by the relative refractive index difference Δ and the core 11 and the cladding 20 of the core 11 used in FIG. 9, further thickness of radius r ₁ and the low-refractive index layer 13 of the core 11 It is the result of calculation which made ratio W / r ₁ with W 0.9. A broken line, the thickness W of the radius r ₁ and the low-refractive index layer 13 with a radius calculated by the relative refractive index difference Δ and the core 11 and the cladding 20 of the core 11 used in FIG. 10, further core 11 The ratio W / r ₁ is calculated to be 0.8. The dotted line, the thickness W of the radius r ₁ and the low-refractive index layer 13 with a radius calculated by the relative refractive index difference Δ and the core 11 and the cladding 20 of the core 11 used in FIG. 11, further core 11 The ratio W / r ₁ is calculated as 0.7.

As shown in FIG. 12, in this embodiment as well, the crosstalk of light in the LP01 mode can be made smaller than −40 dB / km in any case, and the crosstalk of light in the LP11 mode is in any case − The result was more than 30 dB / km. That is, under the above conditions, if the distance between the cores is 32 μm, crosstalk of light in the LP01 mode can be made -40 dB / km or less, and crosstalk of light in the LP11 mode can be made -30 dB / km or more . Furthermore, in FIG. 12, in the C band and L band, the wavelength of light of the LP01 mode is the wavelength with the largest effective core area _Aeff, and the wavelength of light of the LP11 mode is the most effective core area _Aeff. The smaller wavelength is being calculated. Therefore, also in the present embodiment, as in the first embodiment, when LP01 mode light and LP11 mode light are propagated in the same wavelength band, crosstalk of LP01 mode light is set to a small value that does not affect optical communication. There exists an inter-core distance で き る which can make the crosstalk of light of the LP11 mode a large value.

  Therefore, in each of the inter-core distances マ ル チ コ ア of the multi-core fiber 2 of the present embodiment, the crosstalk of light in the LP01 mode (primary LP mode) is -40 dB / km or less, and the light in the LP11 mode (second LP mode) The distance is such that the crosstalk is -30 dB / km or more.

  Also in the multi-core fiber 2 of the present embodiment, for the same reason as the multi-core fiber 1 of the first embodiment, the crosstalk of the LP 01 mode can be improved while suppressing the propagation of light of the LP 11 mode.

Third Embodiment
Next, a third embodiment of the present invention will be described in detail with reference to FIG. 13 to FIG. In addition, about the component the same as that of 1st Embodiment, or equivalent, the same referential mark is attached | subjected and the overlapping description is abbreviate | omitted unless it demonstrates in particular.

  FIG. 13 is a view showing the appearance of the multi-core fiber according to the present embodiment. As shown in FIG. 13, in the multicore fiber 3 of the present embodiment, each core element 10 has a configuration similar to that of the core element 10 in the multicore fiber 1 of the first embodiment. Therefore, also in the present embodiment, each core element 10 is configured to propagate the light of the LP01 mode and the light of the LP11 mode. Further, in the multi-core fiber 1 of the first embodiment, the respective core elements 10 are arranged in a close-packed manner, so the line connecting the respective cores 11 is a triangular lattice, but the multi-core fiber of the present embodiment In 3, the line connecting the cores 11 is a square lattice.

  In the clad 20 of the multi-core fiber 3 of the present embodiment, the distance between the four cores 11 arranged at the outermost side in the clad 20 and the covering layer 30 propagates through the core 11 arranged at the outermost side (LP01 mode (1 Of the LP 11 mode (second-order LP mode) propagating in the outermost core 11 with an excess loss due to absorption of light into the covering layer 30 of the next LP mode being 0.001 dB / km or less The excess loss due to the absorption into the distance is 3 dB / km or more. Furthermore, as in the multicore fiber 1 of the first embodiment, the intercore distance 3 of the multicore fiber 3 of the present embodiment is such that the crosstalk of light of the LP01 mode is −40 dB / km or less, and the light of the LP11 mode is The distance is such that the crosstalk is -30 dB / km or more. For example, the inter-core distance マ ル チ コ ア of the multi-core fiber 3 is 31.8 μm, the cladding thickness Tc is 31 μm, and the diameter of the cladding 20 is 197 μm. In the present embodiment, the distance from the center of the outermost four cores 11 to the outer peripheral surface of the cladding 20 is the cladding thickness.

  Therefore, also in the multi-core fiber 3 of this embodiment, for the same reason as the multi-core fiber 1 of the first embodiment, it is possible to improve the crosstalk of the LP01 mode while suppressing the propagation of light of the LP11 mode.

  FIG. 14 is a side view of the multi-core fiber 3 of FIG. However, in FIG. 14, the covering layer 30 is omitted for easy understanding. As shown in FIG. 14, the multi-core fiber 3 of the present embodiment further includes an extension portion BP which is extended such that the diameter of the plurality of cores 11 is reduced in a part of the plurality of cores 11 in the longitudinal direction. The extending portion BP partially extends the covering layer 30 of the multi-core fiber 3 by heating and pulling the multi-core fiber 3 from the outside of the clad 20.

FIG. 15 is a diagram showing the calculation results of the relationship between the draw ratio and the propagation loss of light of the LP11 mode. In the calculation of FIG. 15, the relative refractive index difference delta _t with respect to the cladding 20 of low refractive index layer 13 was -0.7%, the radius _{r 1} and a ratio _{r 2} of the radius _{r 2} of the inner cladding 12 of the core 11 / R ₁ is 1.7, the relative refractive index difference Δ of the core 11 to the cladding 20 is 0.45%, the radius of the core 11 is 5.17 μm, the radius r _{1 of the} core 11 and the low refractive index layer 13 The ratio W / r ₁ to the thickness W was 0.9. From FIG. 15, the diameter reduction ratio of the extension portion BP to the non-extension portion is set to about 0.6, that is, the diameter of each member of the multi-core fiber 3 in the extension portion BP is the diameter of each member of the multi-core fiber 3 in the non-diameter portion It can be expected that the loss of light of the LP11 mode propagating through the core element 10 will be 10 dB / cm. Therefore, by providing the extension portion BP with a diameter reduction ratio of about 0.6 about 2 cm, it is possible to remove the light of the LP11 mode to such an extent that the light communication is not inhibited. In this case, in the LP01 mode, which is the basic mode, the loss of light due to the extension is, for example, almost 0.001 dB or less, and it hardly affects the optical communication.

  According to the present embodiment, the covering layer 30 not only removes the light in the LP11 mode, but by providing the extension portion BP, the light in the LP11 mode can be further lost, and a mode unnecessary for communication can be obtained. Light can be eliminated more appropriately.

  As mentioned above, although the said embodiment was described to the example about this invention, this invention is not limited to these.

  For example, also in the multi-core fiber 1 of the first embodiment and the multi-core fiber 2 of the second embodiment, the extension part BP provided in the multi-core fiber 3 of the third embodiment may be provided. In this case, in the multi-core fibers 1 and 2, light in the LP11 mode can be further lost, and light in the mode unnecessary for communication can be more appropriately eliminated.

  Further, in the first and second embodiments, 31 core elements 10 are arranged in a close-packed manner, and in the third embodiment, 16 core elements 10 are arranged in a square lattice. However, the number of cores in the multi-core fiber of the present invention is not limited to the above as long as it is plural. FIG. 16 is a cross-sectional view of a multi-core fiber of a modified example of the present invention. In the description of the present modification, the same or similar components as or to those of the first embodiment are designated by the same reference numerals, and redundant descriptions will be omitted unless they are particularly described. As shown in FIG. 16, in the multi-core fiber 4 of this modification, one core element 10 is disposed at the center of the cladding 20 as a first core element, and a plurality of core elements 10 are provided on the outer periphery of the first core element 10. It differs from the multi-core fiber 1 of the first embodiment in that the core element 10 is disposed as the core element of the second layer and the core element is not disposed on the outer peripheral side of the core element 10 of the second layer. Even in such a 1-6 multi-core fiber, the core element 10 is similar to, for example, the core element 10 of the multi-core fiber 1 of the first embodiment, and the inter-core distance 例 え ば is, for example, the multi-core fiber of the first embodiment For the same reason as the multi-core fiber 1 of the first embodiment, the inter-core distance Λ1 of 1 and the clad thickness Tc of the multi-core fiber 1 of the first embodiment are the same, for example. The crosstalk of the LP01 mode can be improved while suppressing the propagation of the light of the LP11 mode.

  In the embodiment and the modification, the multi-core fiber 1 performs single mode communication with light in the LP01 mode in the communication band, and the cores 11 are assumed to propagate light in the LP01 mode and light in the LP11 mode, It was set as the structure which removes the light of LP11 mode. However, the present invention is not limited to this. In other words, it can also be used when performing fu-mode communication or multi-mode communication, and is configured to propagate light up to the 1LP mode higher mode than the mode used by each core for communication; Light may be removed by the covering layer. Specifically, the multi-core fiber of the present invention is a multi-core fiber that communicates with light (x is an integer of 1 or more) up to the x-order LP mode in the communication band, and the plurality of cores 11 are each (x + 1) order The light is propagated to the LP mode, and the inter-core distance Λ is such that the crosstalk of the light to the x-order LP mode is -40 dB / km or less, and the crosstalk of the light of the (x + 1) -order LP mode is -30 dB / km The distance between the core 11 disposed at the outermost position in the cladding 20 and the covering layer 30 is set to a distance equal to or greater than km, to the covering layer of light up to the x-order LP mode propagating in the core disposed at the outermost side. And the excess loss due to absorption of the light of the (x + 1) th-order LP mode propagating in the outermost core is less than 0.001 dB / km or less. Are that distance.

  According to such a multi-core fiber, since each core 11 is a core that propagates 1-mode higher-order light than a core that propagates light to the x-order LP mode, each core 11 transmits to the x-order mode core Containment is enhanced and crosstalk is improved. By the way, the effective core cross-sectional area of light up to the (x + 1) -order LP mode is larger than the effective core cross-sectional area of light of the x-order LP mode. Taking advantage of this, the distance between the outermost core 11 and the covering layer in the cladding 20 is due to absorption of light into the covering layer up to the x-order LP mode propagating in the outermost core. The excess loss is 0.001 dB / km or less, and the excess loss due to absorption of light of the (x + 1) th-order LP mode propagating in the outermost core is 3 dB / km or more it can. Therefore, light of the (x + 1) th-order LP mode unnecessary for communication propagating through the outermost core 11 in the cladding 20 is absorbed by the covering layer 30 and lost. In addition, taking advantage of the fact that the effective core area of the light to the (x + 1) -order LP mode is larger than the effective core area of the light of the x-order LP mode, the inter-core distance Λ is The light crosstalk can be set to a distance of −40 dB / km or less and the light crosstalk of the (x + 1) th-order LP mode to −30 dB / km or more. Therefore, the crosstalk of light to the x-order LP mode used for communication is suppressed, and the light of the (x + 1) -order LP mode, which is light unnecessary for communication, cross-talks. Thereby, the (x + 1) -order light can move to the outermost core 11 in the cladding 20 by crosstalk and is absorbed by the covering layer 30 as described above. Thus, light is propagated up to the x-order LP mode, and the crosstalk of the light up to the x-order mode is improved.

  As described above, even in the case where the multi-core fiber communicates with light in the communication band up to the x-order LP mode, it is preferable that the extension portion BP of the third embodiment be provided. In this case, it is preferable that the loss of light of the (x + 1) th-order LP mode be 20 dB or more, and it is more preferable that the excess loss of light of the x-order LP mode be 0.001 dB or less.

  Moreover, although it was set as the structure where each core 11 is surrounded by the low refractive index layer 13 in the said embodiment and modification, this invention is not limited to this. For example, each core 11 may be directly surrounded by the cladding 20. Even in this case, each of the cores 11 propagates the light to the (x + 1) -order LP mode, and the inter-core distance Λ indicates that the crosstalk of the light to the x-order LP mode is -40 dB / km or less And the crosstalk of light of the (x + 1) order LP mode is −30 dB / km or more, and the distance between the core 11 disposed at the outermost side in the cladding 20 and the covering layer 30 is disposed at the outermost side The excess loss due to absorption of light to the covering layer 30 up to the x-order LP mode propagating through the core 11 is 0.001 dB / km or less, and the (x + 1) -order LP mode propagating through the core 11 disposed outermost It is possible to design such a distance that the excess loss due to the absorption of the light into the covering layer 30 is 3 dB / km or more.

  EXAMPLES Hereinafter, the contents of the present invention will be more specifically described with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1
The multi-core fiber 1 of the first embodiment was created. The average value of the inter-core distance マ ル チ コ ア of the created multi-core fiber 1 is 31.6 μm, the average value of the cladding thickness Tc is 31.5 μm, the average value of the cladding diameters is 231.0 μm, and the average of the outer diameter of the covering layer 30 is The value is 334.8 μm. The length was 11.2 km. The propagation loss of the LP01 mode light propagating through the multi-core fiber 1 was measured. In addition, polarization mode dispersion PMD, polarization loss difference PDL, effective core cross-sectional area _Aeff , and cable cutoff wavelength λc were measured. The results are shown in Table 3. In Table 3, 1 to 12 of the cores indicate the fourth layer core in the first embodiment, 13 to 24 of the cores indicate the third layer core in the first embodiment, and 25 to 30 of the cores The 2nd-layer core in 1st Embodiment is shown, 31 of a core shows the 1st-layer core in 1st Embodiment.

  The loss of light propagating through the core includes losses other than excess loss due to absorption into the coating layer. Therefore, it is considered from Table 3 that the excess loss due to absorption into the coating layer of the light of the LP 01 mode propagating 1 to 12 of the core is 0.001 dB / km or less.

Next, crosstalk of the cores 11 adjacent to each other was measured. The crosstalk of the light of the LP01 mode was measured by calculating from the wavelength dependency of the crosstalk of the light of the LP01 mode in the band above the cutoff wavelength. In the measurement of crosstalk of light in the LP11 mode, light from the wavelength variable light source is converted into light in the LP11 mode by the mode converter and is incident on a specific core, and a two-mode measurement core adjacent to the core is An optical fiber was connected, and it carried out by receiving and calculating the light radiate | emitted from the said 2 mode optical fiber. The results are shown in Table 4. The cores described in the first column of Table 4 and the cores described in the second column are cores adjacent to each other.

  As shown in Table 4, crosstalk of light in the LP11 mode is larger than crosstalk of light in the LP01 mode or crosstalk between light in the LP01 mode and light in the LP11 mode, while suppressing crosstalk in the LP01 mode, As a result, light of LP11 mode can be moved to the outermost core by crosstalk.

(Example 2)
The multi-core fiber 2 of the second embodiment was created. The average value of the inter-core distance マ ル チ コ ア of the created multi-core fiber 2 is 32.1 μm, the average value of the cladding thickness Tc is 31.2 μm, the average value of the cladding diameter is 230.8 μm, and the average of the outer diameter of the covering layer 30 is The value is 337.0 μm. The length was 10.5 km. The crosstalk of the cores 11 adjacent to each other was measured in the same manner as in Example 1 for this multi-core fiber 1. The results are shown in Table 5. The cores described in the first column of Table 5 and the cores described in the second column are cores adjacent to each other.

  As shown in Table 5, crosstalk of LP11 mode light is larger than crosstalk of light of LP01 mode and crosstalk of light of LP01 mode and light of LP11 mode, while suppressing crosstalk of LP01 mode, As a result, light of LP11 mode can be moved to the outermost core by crosstalk.

(Example 3)
The multi-core fiber 4 of a modification was created. The average value of the inter-core distance マ ル チ コ ア of the created multi-core fiber 4 is 32.4 μm, the average value of the cladding thickness Tc is 29.9 μm, the average value of the cladding diameter is 124.4 μm, and the outer diameter of the covering layer 30 is average The value is 220.0 μm. The length was 10.0 km. For this multi-core fiber 4 in the same manner as in Example 1, propagation loss of light of LP01 mode and light of LP11 mode, polarization mode dispersion PMD, polarization loss difference PDL, effective core area A _eff , cable cutoff The wavelength λc was measured. The results are shown in Table 6. In Table 6, 1 to 6 of the cores indicate the cores disposed on the outer peripheral side, and 7 of the cores indicates the core disposed at the center of the cladding.

  It is considered from Table 6 that the excess loss due to absorption of light of the LP01 mode propagating from 1 to 6 in the core into the coating layer is 0.001 dB / km or less.

  From the results of the above examples, according to the multi-core fiber of the present invention, it was confirmed that crosstalk of LP01 mode light is suppressed and crosstalk of LP11 mode light occurs. Further, in the above example, it is considered that the excess loss due to the absorption to the covering layer of the light disposed in the outermost part from the propagation loss of the light of the LP01 mode and propagating in the core becomes 0.001 dB / km or less. In the case of measuring the propagation loss of light, it is considered that the excess loss due to absorption of the light propagating through the core disposed at the outermost side into the covering layer is 3 dB / km or more.

  As described above, according to the present invention, a multi-core fiber capable of improving the degree of freedom in design is provided and can be used in the field of optical communication.

1 to 4 ... multicore fiber 10 ... core element 11 ... core 12 ... inner cladding 13 ... low refractive index layer 20 ... cladding 30 ... coating layer BP ... extended portion Tc ・ ・ ・ cladding thickness ・ ・ ・ distance between cores

Claims (5)

A multi-core fiber that communicates with light (x is an integer of 1 or more) up to the x-order LP mode in the communication band,
With multiple cores,
A cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores;
A covering layer covering the cladding and having a refractive index higher than that of the cladding;
Equipped with
Each of the plurality of cores propagates light up to (x + 1) order LP mode,
The inter-core distances are such distances that crosstalk of light to x-order LP mode is -40 dB / km or less and crosstalk of light of (x + 1) -order LP mode is -30 dB / km or more,
The distance between the outermost core in the cladding and the covering layer is the excess loss due to absorption of light into the covering layer up to the x-order LP mode propagating in the outermost core. The distance is 001 dB / km or less, and an excess loss due to absorption of the light of the (x + 1) order LP mode propagating in the outermost core into the covering layer is 3 dB / km or more. Multicore fiber. It further comprises an extension portion which is extended such that the diameter of the plurality of cores is reduced in a part of the plurality of cores in the longitudinal direction,
2. The multi-core fiber according to claim 1, wherein the plurality of cores propagate light up to an x-order LP mode in the drawing section, and propagation of light of an (x + 1) -order LP mode is suppressed. The multi-core fiber according to claim 2, wherein the loss of light of (x + 1) order LP mode is set to 20 dB or more in the extension section. The multi-core fiber according to any one of claims 1 to 3, wherein x = 1.   Each of the plurality of cores is surrounded by a low refractive index layer having a lower refractive index than the refractive index of the cladding.
The multi-core fiber according to any one of claims 1 to 4, characterized in that:

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